A Primer on How A Pressurized Water Reactor Shuts Down

My motivation for this layman's primer is to correct the misstatements of what actually goes on when an operating Pressurized Water Reactor Shuts Down as a result of either a suddenly occurring event such as an earthquake or because of an impending severe storm such as a hurricane. There are two types of shutdowns to be considered: (1) one in which an event such as earthquake or loss of offsite power occurs and the unit automatically trips, and (2) one in which station procedures recognizing a deteriorating weather situation (as an example) takes a slower path to shutdown. I will discuss both of these cases. I will discuss the equivalent case for an operating Boiling Water Reactor in a separate blog.

Safety Considerations
The key difference between a nuclear power plant and a coal fired thermal power plant is that after a unit trip the nuclear heat source continues to produce heat. When a coal fired power plant is tripped: the blower fans and fuel supply is cut off quickly stops generation of new heat. The heat that remains is a result of the hot tubes in the boiler needing to cool down and this is accomplished by continuing to supply feedwater to the boiler. A nuclear power plant on the other hand continues to generate decay heat. This is a well known and well understood phenomenon and is characterized by design standards such: ANSI/ANS Std. 5.1 [Reference 1]. A graph of how decay heat drops off with time (dotted line) is shown in the figure below.

The dotted lines are taken directly from the standard. One second after a reactor trips, the standard assumes the heat is at 8% of the original power output. So if we had a reactor generating 1000MegaWatts of heat at one second after trip it is putting out 80 MegaWatts. At 100 seconds the heat has dropped to ~3.5% (or like 35MegaWatts) and so forth. As in the case of the coal fired boiler - after a unit trip there are is a lot of very hot metal which need to be cooled down as well (e.g. piping, steam turbines, etc). All of this requires continuous addition of water.

Plant Trip from Full Power with Offsite Power Available
When a pressurized water reactor (see figure below) is operating at full power one would find: hot steam is being produced at nominally 900-1000 pounds per square inch or "psi" by boiling water in the steam generators. The source of heat in the steam generators is hotter water, typically 580-600F pumped to the steam generators from the reactor by reactor coolant pumps that take the water from the colder side of the steam generators.

Depending on the size and rating of the reactor plant, water coming into the reactor enters at above 500F and is heated in the reactor and heads for the steam generators. There are two main sources of heat to the water: heat transferred from the reactor (depending on the rating of the unit several hundreds to thousands of MegaWatts) and heat added to the water by the functioning of the reactor coolant pumps (which can be several MegaWatts). When a unit trips from full power, the control rods are inserted in 1-2 seconds and the reactor transitions from heat directly from fissioning to decay heat plus reactor coolant pump heat. The figures below are taken from a actual full power plant trip as recorded by the unit's plant data logging computer..

When the measured neutron "flux" power (scale on the right) abruptly drops - the actual time when the reactor trip occurred - the measured temperatures at the cold and hot sides start to approach each other. The difference between the upper and lower temperature is proportional to the decay heat and heat added by the reactor coolant pumps.

The immediate actions by a well trained crew of operators in the control room would be to announce over the plant address system that the reactor had tripped and then they begin their immediate reviews to confirm that: all control rods have fully inserted, that the turbine has tripped and main generator disconnected, that electrical buses needed to power equipment is energized, and that pressures, water levels and temperatures are trending the way that is expected and that there is no need for operating emergency cooling systems. Equipment operators in the plant will typically walk to their pre-defined post-unit-trip duty stations and make further adjustments on equipment in the turbine building. The normal preferred pathway from this point on would be to continue to generate steam in the steam generators - send it directly to the main condenser - cool the steam and feed it back to the steam generators. After about 10 minutes in this condition, the control room staff would actually hold a meeting to discuss what caused the trip, any out of tolerance conditions observed by the operators, or indications that are not as expected, and where to proceed from this point based upon their written procedures. The options include to continue to remain on steam generator cooling (which we call hot standby) or proceed to cold shutdown - meaning the primary coolant system in further cooled down to temperatures where heat is removed by a special set of heat exchangers called a residual heat removal system.

Plant Trip from Full Power with Loss of Offsite Power
In a plant trip from full power without offsite power (like recently occurred at the North Anna plant following the earthquake in Mineral Virginia last week) the response by the control room operators would essentially be the same. The initial steps are in fact identical. Typically they would experience a momentary dimming of control room lighting while emergency diesel generators kicked in. They would announce a plant trip, confirm the control rods are inserted, that the turbine and main generator have tripped, and evaluate the status of incoming electrical connections.

There are several key differences in what equipment would be used to remove post-trip decay heat. All non-essential electrical buses are de-energized when there is a loss of offsite power. Emergency diesel generators have started and essential pumps, motors and loads such as battery chargers are reloaded automatically to the diesels. The main condenser unit - which is not a safety system and is not powered by emergency diesel generators - and all of its associated pumps and heat exchangers are just sitting there and not available to remove decay heat from the steam generators. Additionally the main reactor coolant pumps which are not safety related de-energize and spin down. While this implies no forced coolant flow -- it also means a rapid elimination of the several MegaWatts of pump heat added to the system - leaving only the decay heat from the reactor. Initially, there is considerable momentum of water circulating in the coolant system to carry away the decay heat from the reactor.

So, after this point: How is the heat transported from the reactor to the steam generators?

The answer is natural circulation. Pressurized Water Reactors (both those with U-tubes, and those with straight tubes) are designed with relatively large vertical elevations in mind that allow hot water to rise - as the coolant flows up through the reactor to the steam generator. As the water cools - via transferring heat in the steam generators through boiling water on the steam side it becomes colder and denser. The denser, heavier water wants to flow downwards to the lowest point in the coolant system -- which in this case is the bottom of the reactor vessel. I show two elevations drawings for common nuclear power plant designs below. The points being highlighted are the "thermal centers" in the steam generator (Hsg) and the reactor (Hrx). The thermal center is the point in the closed loop of the coolant system where the mid-range coolant temperature -- which we call Tavg is physically located.  As long as: Hsg is elevated above Hrx water is naturally circulated to remove decay heat without any moving parts in the primary coolant system.

So - hotter less dense water rises in the reactor core, and colder denser water wants to fall downwards - as long as heat is being removed in the steam generators. I show below a trend plot below from an actual trip with loss of offsite power.

In the first few hundred seconds, what we see is the rapid drop off of forced primary flow provided by the pumps -- but it doesn't go to "zero" because natural circulation kicks in and provides 4-5% normal flow. As the decay heat from the reactor core starts to drop off, the heat added to the water drops off and this naturally slows down the rate of water flow. The figure below shows what the temperature trend looked like over a significantly longer time scale as the plant entered a stable long term natural circulation cooling..

I have previously mentioned that it is important to keep water flowing to the steam generators and steam flowing out of them.

Getting water into the steam generator is accomplished by what we call an auxiliary feedwater (AFW) pump system. Depending on the design, this can be a combination of one (or two) electrically driven AFW pumps and a diverse steam driven AFW pump. Typically only one of these is needed and operators would keep only one feeding the system and shutdown (or secure) the others to keep them in reserve and to prevent them from overfilling the steam generator. The electric driven AFW pump(s) would be powered by a diesel. The steam driven pump is powered by a small portion of the steam generated by the steam generators. This is kind of a "play as you go" operation - the higher the decay heat load the higher the steaming rate - the lower the decay heat to lower the steaming rate. These pumps take suction from a safety related water storage tank (or in many designs several tanks) with enough water to last typically a day. If this tank should become depleted operators have that whole day to align to an alternate water source such as a small diesel fire water pump using well water, river water, or even ocean water.

(Photo of a steam driven, Terry Turbine, auxiliary feedwater pump with Woodward Governor)

Getting steam out of the steam generators is accomplished by venting the steam to the atmosphere either by manually opening an atmospheric dump valve or letting the steam pressure naturally rise to the point where a steam generator relief valve operates. Venting to the atmosphere is necessary because - recall: the main condenser is unavailable due to lack of power for running pumps. Venting to the atmosphere is simple and straight forward but it results in continuous depletion of water supplies and this is the main reason for large water storage tanks and the ability to make up to those tanks with alternate water sources such as fire water.

 (photo of the pneumatic controls on a steam generator atmospheric dump valve)

Throughout the cool-down (or until offsite power is restored), electrical power would be provided by redundant diesel generators typically rated at 4-5MegaWatts and about the size of a locomotive engine. These units provide all necessary power to run electric AFW pumps (if operators chose to run an electric pump), power other essential cooling water pumps to cool pump bearings, run safety related room cooling, and keep the redundant batteries powered so that the control room instruments remain powered and operable. To keep the diesels running all that is required is fuel oil and some form of engine cooling. Some plants have water cooled diesels others have air cooled diesels. Fuel oil is stored in a smaller quantity for immediate operational needs in what is called a day tank and over longer periods would be replenished by a fuel oil transfer pump from a much larger supply tank.

With the plant temperatures stabilized - typically in about 10-20 minutes - the control room operators would make contact with outside dispatchers to determine the source of the loss of offsite power and when it was likely that offsite power connections would be restored. From this information the crew meeting would be held and following plant procedures, decisions would be made on whether to remain in hot standby or to proceed to cold shutdown.

What is Different if the Plant has Advanced Warning?
A Hurricane like we are experiencing this weekend on the east coast is an example of an external event which has many hours of advanced warning. The typical issues that might warrant a shutdown of the plant before the hurricane reaches the plant could include the likelihood of storm related damage to the main switch-yard or incoming electrical transmission system. High winds and debris (tree branches, etc.) can knock down transmission lines. Hurricanes have been known to transport large amounts of salt on to high voltage electrical insulators causing transmission lines to electrically short-circuit to transmission tower metal and then to ground. Hurricanes have also been known to tear up the seaweed and kelp from the ocean bed and cause clogging of intake structures - and this could result in reduced water flows to the diesel (if it isn't air cooled) or to other heat exchangers needed to support operation of essential systems. Utilities and the people who operate nuclear power plants know of these potential challenges and have specific emergency operating procedures to deal with external events with long lead times. The general idea behind these procedures is to reduce power ahead of time (thus starting off with a lower decay heat level) and making use of their largest single heat removal system available to them -- namely: the main condenser. Because the condenser is relatively large, it is possible to quickly cooldown the steam generators and reactor coolant system while not discharging steam to the atmosphere. This means the water in the storage tanks for the AFW pumps remains at a maximum inventory level for as long as possible. [I would point out that nuclear power plants also have written procedures covering advance warning of tornadoes and high winds. Although these scenarios may have only 10 minutes advance warning these would also be addressed by rapidly reducing power.]

Typical utility procedures in the event on an oncoming hurricane (where there could be an extended loss of offsite power) would include:

• reducing overall power output levels several hours before the storm arrives
• topping off the reserve diesel fuel oil tanks
• securing from any non-essential maintenance activities that might leave the plant vulnerable in a loss of offsite power situation
• doing a walk-down of plant areas to remove or secure outside equipment or supplies in the plant yards that might become a missile hurled at plant equipment by hurricane force winds
• installing temporary flooding protection to doorways leading into plant buildings
• calling in and stationing additional plant equipment operators in specific areas that might need additional monitoring during a hurricane.

It is thus not surprising that given decades of coping with actual hurricanes, tornadoes, earthquakes, continuous operator training (roughly a week every five weeks involving classroom refresher training and simulator drills on what to do), well written procedures and guidance -- that US nuclear power plants know how to cope with severe weather and other natural disasters. When I read in the main street media that nuclear power plant operators don't know how to deal with such situations and that further improvements are urgently needed because a theoretical physicist thinks we just "dodged a bullet"..... I have only one word to say, and that is:

"  B@!!$'t  "

References
[1} Decay Heat Power in Light Water Reactors, ANSI/ANS Std. 5.1-2005, issued by the American Nuclear Society.

Recalling the June 24th 1978 Boston Globe "Pro-Seabrook Ad"

In June of 1978 a group of us at Combustion Engineering Nuclear Power Division following extensive news media coverage of the planned demonstrations by the Anti-Nuclear Clamshell Alliance decided we should make a counter statement in the news media. We hit upon the idea of taking out a full page advertisement in the largest circulation newspaper in the Boston - Southern New Hampshire area - the Sunday Boston Globe. It was strange effort. Here we were in Connecticut planning a media event that would mainly benefit a competitor: Westinghouse Electric in Pennsylvannia and a power company in New Hampshire (Public Service of New Hampshire). The text was primarily drafted by Bill Burchill. Uli Decher and I did some minor editing, but Bill was the main author. Then we went to work on collecting money. Within several days (and this was significantly before the internet and "Pay-Pal") we had collected several thousand dollars from over 750 like minded folks. 

The ad below appeared in the Sunday morning June 24th, 1978 edition of the Boston Globe at a time when the majority of the Clamshell Alliance were confined in makeshift jails after being arrested and refusing to make bail.

_______________________________________________

SEABROOK

A Demonstration Against Nuclear Power?
OR A Demonstration Against Established Society?

Have you thought about, truly examined, the benefits of nuclear power? Are these benefits consistent with our structure of society and its values? Does denial of these benefits oppose our structure of society? Do the demonstrators oppose nuclear power, or do they oppose our structure of society? What is the real issue?!?

What are the benefits?

The benefits of nuclear power can be shared by everyone. But, denial of these benefits will most heavily impact the economically disadvantaged people in our society. The benefits for everyone are clearly represented by the following quotations from the NAACP policy on energy issued in December 1977, and reaffirmed in April 1978:

"Since the early 1960's gains have been made toward bringing the nation's Black citizens into the mainstream of American Economic Life. This has occurred largely during a period of expansion in the economy which created new opportunities for jobs. However, a great deal more remains to be done. We still have tremendous unmet social and economic needs....An abundant energy supply will be necessary if we are to have any chance to meet these challenges.....All alternative energy sources should be developed and utilized. Nuclear power, including the breeder, must be vigorously pursued because it will be an essential part of the total fuel mix necessary to sustain an expanding economy.....We recognize that nuclear power does present certain problems. But we think these problems can be solved through the dedicated efforts by government, the scientific community and industry working cooperatively together. Notwithstanding the claims of opponents of this source of energy, the fact is that nuclear power will be required to meet our future needs for electricity. If we do not move ahead now with nuclear energy, the next generation is likely to be sitting around in the dark blaming the utilities for not doing something this generation's officials would not let them do."

Who opposes these benefits?

Many of those who demonstrate against nuclear power are disillusioned with American society and distrustful of its institutions. To them nuclear power symbolizes life's frustrations (so big, so complex, it can't be understood) and its elimination is seen as one bridge to their desired social changes. Many leading opponents of nuclear power advocate the requirement for radical social changes.

Many people have an honest and sincere concern over safety and possible proliferation of materials for nuclear weapons. We would be forever judged as totally negligent in our obligation to the preservation of humanity if we did not consider these concerns. More personally, we too have families. HOWEVER,WE HAVE EXAMINED NUCLEAR POWER AND WE HAVE DECIDED IN ITS FAVOR.

What is the demonstrators' real goal?

For many, the real goal is a major change in American society. Nuclear power is not a central issue itself, but rather the clamor against it is a tool, a lever to be applied in creating an upheaval of our social, economic, and political patterns of life. An aspiration for seeking change is stated to be a desire for a more democratic society. However, the large-scale institutions which are a key product of our free enterprise economic structure are somehow excluded from this society. They are to be replaced instead with small-scale, localized technology controlled by neighbors and friends.

What are the consequences?

The consequences are evident from the history of failures of utopian experiments. Mao's "great leap forward" wherein technology was forced toward backyard industries, including even steel smelting, was a notable failure of an enticing dream. Mahatma Ghandi's "cottage industries" are held by many to have retarded progress toward improving the lot of the masses. One of the greatest improvements for the Indian village inhabitants was electrification. This step parallels the dramatic improvements to our standard of living brought about by the Rural Electrification Agency in the United States in the 1930's. Many of today's "nuclear opponents" have no personal knowledge of that period. Neither do they know what can be made available only by using the large-scale institutions which they wish to abolish.

What should they do?

The "nuclear opponents" should be honest with us about their real goals and use the established democratic processes to seek those goals. Rather than seeking referenda against nuclear power, let them ask for votes on the consequences of the unavailability of that power. Convince the over two million American workers laid off due to energy shortages last winter that there is no connection between jobs and energy supply. Convince the poor, the minorities, the working-class that America is so rich that it no longer needs growth because economic needs are no longer the principle concern of its people. Convince the people who are "without" that they don't want washing machines and refrigerators to relieve domestic drudgery, that they don't want cars for freedom on weekends and holidays, that they don't want the comfort of central heat in the winter and air conditioning in the summer.

What should we do?

We should recognize the real social issues even if they are disguised as oppositon to nuclear power. We should insist upon an open and fair evaluation of the consequences of actions both for and against nuclear power. Finally we should realize the true effects which our decisions will have on the future of our society.

_________________________________________

So what has changed over the years?

The Seabrook Station Nuclear Power Plant went into commerical service after all the regulatory and licensing delays in 1990 - some thirteen years after recieving a construction permit. One of the last hurdles was when then Gov. Michael Dukakis refused to agree to Massachusetts participating in federally required emergency planning. Seabrook Station today generates 1245MWe. This is enough energy to supply power to over 900,000 homes and businesses. It has a workforce of approximately 1100 employees and contributes ~$20million to the local economy including ~$10million in property taxes. The current owners NextEra Energy (Florida Power & Light) have submitted their applications to operate plant until 2050.

The Chinese and Indians having abandoned the "great leap forward" and cottage industries are building new nuclear power plants and pioneering methods of speedier construction such as modularized portions of buildings being constructed off-site in factories and then shipped for assembly at the site.

Radioactivity Release from Natural Gas Production

Imagine if you can a nuclear power plant releasing radioactive materials to the environment at levels hundreds of times greater than Federal Drinking Water Standards - and the responsible federal authority responded that they were unaware of any releases. In the case of our Environmental Protection Agency (EPA) that's exactly what is going on.

“I’m not aware of any proven case where the fracking process itself has affected water.”

Statement by EPA Administrator, Lisa Jackson

Responding to questions before the U.S. House Oversight Committee

May 2011 [Reference 1].

I am guessing that with all the ongoing efforts to write new "Clean Water Rules" aimed at shutting down  power plants that have "once through cooling systems" (rather than cooling towers) and discharge warm water that the Obama Adminstration's EPA Head didn't have time to read the New York Times series on "Drilling Down". [Reference 2].

I found the graphic below from the New York Times particularly informative.

Part of Ms. Jackson's deliberate ignorance of what is going on...... is actually a result of Congressional intent at the urging of certain oil and gas interests. The Energy Policy Act of 2005 exempted fracking from EPA regulations under the Safe Drinking Water Act. Its not a new development either. This monkey business has been going on for years.

Congress passed the Resource Conservation and Recovery Act (RCRA) in 1976 as an amendment to the Solid Waste Disposal Act of 1965 in an effort to enact more comprehensive waste disposal standards nationwide. Through RCRA, Congress declared that the “disposal of solid waste . . . without careful planning and management [was] a danger to human health and the environment.” Congress later amended RCRA with the Solid Waste Disposal Act Amendments of 1980. One of the 1980 amendments, the so-called Bentsen and Bevill Amendments, temporarily exempted “drilling fluids, produced waters, and other wastes associated with the exploration, development, or production of crude oil or natural gas” from regulation under RCRA.Under the Bentsen Amendment, Congress directed EPA to conduct a study to determine whether or not drilling and production wastes should be regulated as hazardous wastes under RCRA. The studies continue [Reference 3] and the best option put forth -- to the pleasure of the oil and gas industry is dilution -- by a process called "Landspreading" which is basically spreading the contamination around over large areas of land.

What is the Source of this Radioactive Contamination?

Our earth is naturally radioactive and is already heavily laden with naturally occurring radioactive isotopes of Uranium, Thorium, Radium, Radon. The figure below from the US Geological Survey shows the relative abundance of several of these naturally occurring isotopes.

Far from being rare elements, Uranium and Thorium are as abundant as common Nickel. Theses elements and their decay products can be bound up in the rock for millenia, slowly decaying back to Lead while emitting alpha, beta, and gamma rays. This continuous decay is one of the heat sources of geothermal energy. But, when drilling deep into the earth's crust, it is possible to run into natural deposits of such ores and their radioactive decay products such as Radium and Radon. Drilling for oil and natural gas is accomplished by pulverizing the underlying rock with heavy rotating drills and flushing the materials out of the exploratory well to the surface using drilling mud. So the drilling mud can come to the surface laden with Uranium, Radium, and Thorium.

As noted by the US Geological Survey in Reference 4:

"In 1989 the American Petroleum Institute sponsored a preliminary nationwide reconnaissance of measureable radioactivity at the exterior surfaces of oil-field equipment. The results of this non-statistical sampling indicated that gamma-ray radiation levels exceeded natural background radiation levels at 42% of the sites. Radiation levels greater than five times the median background of all site were found at approximately 10% of the sites. Most of the sites with markedly higher radioactivity were concentrated in specific geographical areas such as the Gulf Coast, northeast Texas, southeast Illinois, and south-central Kansas. Additional surveys by some state agencies identified radioactive oil-field equipment in northern Michigan and eastern Kentucky. Pipe casings, fittings, and tanks that have an extended history of contact with produced water are more likely to contain radioactive deposits than other parts of the plumbing system at oil-field production sites. Soil in the immediate vacinity of production sites may be unusually radioactive."

The problem then becomes: What to do with this contaminated equipment and soil? Its contaminated. Anyone working in or around the mud can become contaminated. At the end of the day, the workers on the drill rigs hop in their pick-up trucks and head to town -- where the material gets further spread around.

Radiological Effects of NORM
This type of radioactive drilling waste material goes by the accronym of "NORM" -or- Naturally Occurring Radioactive Material. NORM is frequently found in settling ponds near drilling sites, inside of fluid tanks, on drills, and drill structures. The magnitude of human exposure depends on the relative Radium and Uranium concentration of the NORM. As a point of comparison: depending on where you live your background radiation dose can be 200 - 500 mR/yr. The figures below taken from Reference 3 show the projected doses one obtains as a function of the activity per gram of contaiminated drilling mud and the increased risk of latent cancer.  From this figure we note that any mud with concentrations above ~100pCi/g are going to result in doses that are significantly above normal background radiation levels -- hence the need to dilute the drilling mud by "Landspreading" it.

What I found interesting in the study was that having a home built over "Landspread" NORM results in higher risks than the risks to the workers who did the Landspreading.

My Take on All of this ?

I work for an electric company that operates nuclear power plants. I just finished my day-long Radiation Worker requalification training. I had to pass a 100 question test on: radiation effects, federal dose limits for workers and the general public, proper use of dosimetry, how to prevent the spread of microscopic quantities of radioactive materials out of the plant, how to use a personal frisker and properly exit the radiation control check point, how to properly use a radiation work permit, how to don Anti-Cs, proper As Low As Reasonably Achievable (ALARA) practices, chemistry controls, costs of low level waste, and why as an industry we focus on everything to minmize the spread of contamination. 

Then I see the oil and gas industry mishandling radioactive materials and pretending there is no radioactive contamination, and thus no need for dosimetry, ignoring worker exposure, transporting contaminated drilling equipment from state to state, and when the drilling at one site is completed their solution to dealing with radioactively contaminated mud is to "Landspread" it. On top of this, we have politicians who protect the oil and gas industry by exempting them from dealing with their radioactive waste disposal by exemptions, and federal regulators in charge of environmental protection pretending nothing is going on.

So much for the clean energy from America's Natural Gas.

References:
[1] Dr. Robert Peltier, PE, "Fracking Problems", Power Magazine, August 1, 2011

[2] "Toxic Contamination from Natural Gas Wells", New York Times, February 27, 2011.

[3] K.P.Smith, D.L.Blunt, J.J. Arnish, "Potential Radiological  Doses Associated with the Disposal of Petroleum Industry NORM via Landspreading", DOE/BC/W-31-109, Final Report, September 1998.

[4] "Naturally Occurring Radioactive Materials (NORM) in Produced Water and Oil-Field Equipment - An Issue for the Energy Industry", US Geological Survey Fact Sheet FS-142-99, issued September 1999.